Underfill materials for circuit board assembly are adhesives that are employed for the attachment of devices, such as flip chips, to electronic substrates. These adhesives provide a means for the mechanical connection between the device and the substrate which relieves stress on the solder joints and improves the reliability of the assembly. Voids in the underfill layer are problematic because compressive forces can cause extrusion of solder into the void regions. If the void spans the area between adjacent solder joints, solder extrusion can ultimately cause an electrical short. C-mode scanning acoustic microscopy, C-SAM, can be used for the detection of these voids, but the process is relatively slow and requires submersion of the sample in water. Therefore, this is not an attractive method for the inspection of underfilled devices in a manufacturing environment because the water bath is a potential source of ionic and microbial contamination. X-ray analysis of underfilled assemblies is not a viable option in most cases because the degree of contrast between the void regions and the underfill is typically quite low. Conventional methods for increasing contrast by the addition of inorganic oxides such as Y2O3 are not practical for either capillary or no-flow (self-fluxing) underfills. Capilally underfills are typically heavily filled, and the addition of more filler in the form of radio contrast agents should have a negative impact on viscosity. Furthermore, uniform dispersion of the radio contrast agent provides additional challenges that must be met. No-flow underfills are typically unfilled because filler particles in the underfill could be incorporated into the solder joints. Therefore, incorporation of conventional metal oxide particles is not a viable solution for increasing the X-ray opacity of no-flow underfills.